1. Field of the Invention
This invention relates to a displacement information measuring apparatus, and particularly is suitable for measuring the displacement information of a moving object or fluid (hereinafter referred to as the "moving object") in non-contact by detecting the shift of the frequency of scattered light subjected to Doppler shift in conformity with the movement speed of the moving object.
2. Related Background Art
As an apparatus for measuring the displacement information of a moving object, in non-contact, and highly accurately, use has heretofore been made of a laser Doppler velocimeter and a laser encoder (an optical type displacement information measuring apparatus). In the laser Doppler velocimeter, the movement speed of a moving object is measured by the utilization of the effect of applying a laser beam to the moving object and the frequency of scattered light by the moving object shifting in proportion to the movement speed (Doppler effect).
Such laser Doppler velocimeters are proposed, for example, in Japanese Laid-Open Patent Application No. 2-262064, Japanese Laid-Open Patent Application No. 4-230885, Japanese Laid-Open Patent Application No. 8-15435, etc. In these velocimeters, the detection of the direction of movement of a moving object is not effected and there has been the tendency of detection becoming difficult when the speed of the moving object is approximate to 0.
In the laser Doppler velocimeter, a method of installing a frequency shifter using a flat plate of electro-optical crystal (hereinafter referred to as the "electro-optical element") in the optical path of two light beams, giving a predetermined frequency difference between the two light beams by the frequency shifter and causing the light beams to enter a moving object, thereby accurately detecting the direction of movement and movement speed of the moving object even when they are approximate to 0 has been published by Foord et al. (Appl. Phys., Vol. 7, 1974, pp. 136-139).
FIG. 1 of the accompanying drawings is a schematic view of the essential portions of a laser Doppler velocimeter utilizing the principle of detection described above. In FIG. 1, the reference numeral 10 designates a frequency shifter comprised of two electro-optical elements 10a, 10b and a driving circuit 20 therefor.
A light beam of a wavelength .lambda. from a light source 1 is made into parallel light 3 by a collimator lens 2, and the parallel light 3 is separated into two light beams 5a and 5b by a beam splitter 4, and the respective light beams are reflected by mirrors 6a and 6b and incident on the electro-optical elements 10a and 10b constituting the frequency shifter 10.
At this time, the light beams 5a and 5b are subjected to frequency shift by the sawteeth wave voltage driving (serodyne driving). Thereby a frequency difference is imparted between the two light beams 5a and 5b, and the two light beams are incident on the surface of a moving object 7 moving at a speed V.sub.0 in the direction of arrow so as to intersect with each other. Of the light beams which have been incident, scattered light created from the moving object 7 is condensed by a condensing lens 8 and is directed to a photodetector 9, whereby a Doppler signal is obtained from the photo-detector 9.
At this time, the frequencies of the scattered lights by the two light beams are subjected to Doppler shift in proportion to the movement speed V.sub.0 and interfere with each other on the detecting surface of the photodetector to thereby bring about a change of lightness and darkness. The frequency of the lightness and darkness at this time, i.e., Doppler frequency F, is as follows by the frequency difference fR between the two light beams: EQU F=2.multidot.V.sub.0 .multidot.sin(.theta.)/.lambda.+f.sub.R(1)
Thus, even when the speed V.sub.0 of the moving object 7 is low, for example, approximate to 0, it can be measured by setting the frequency difference f.sub.R with a suitable value, and the direction of speed can also be measured at the same time. This construction is a form utilized chiefly as a current meter.
Generally, when a light beam of high coherency such as a laser is applied to an object, scattered light created by the minute unevenness of the surface of the object is subjected to random phase modulation and forms a spot pattern, i.e., a so-called speckle pattern, on an observation surface. In a laser Doppler velocimeter, when a moving object moves, a change of lightness and darkness by the Doppler shift on the detecting surface of a photodetector for the detection of scattered light is modulated by an irregular change in lightness and darkness by the flow of the speckle pattern, and an output signal from the photodetector is modulated also by a change in the transmittance (or the reflectance) of the object to be examined.
Generally, in the laser Doppler velocimeter, the frequency of the change in lightness and darkness by the speckle pattern and the frequency of the change in the transmittance (or the reflectance) of the moving object are low as compared with Doppler frequency. Therefore, use is made of a method of passing the output from the photodetector through a high-pass filter to thereby electrically eliminate a low frequency component and take out only a Doppler signal.
However, when the speed of the moving object is low and the Doppler frequency becomes low, the frequency difference between it and a low frequency fluctuation component becomes small and there arises the problem that a high-pass filter cannot be used and it is difficult to measure the displacement information of the moving object with good accuracy. Further, the direction of speed cannot be detected in principle.
In contrast, the aforedescribed method (frequency shifter) published by Foord et al. attaches a predetermined frequency difference to two light beams before the two light beams are applied to the moving object, and makes the measurement of the moving object including the stationary state and the direction of speed thereof possible.
However, in a velocimeter utilizing the frequency shifter, the measurement of the moving object including the stationary state and the direction of speed becomes possible, while the optical system thereof becomes complicated and this has led to the tendency toward the bulkiness of the entire apparatus.